Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording system attracts attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, the dye diffusion transfer recording system has the following advantages over silver halide photography: that is, the system is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing a dye(s) is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dye(s) contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
With the spread of such the dye diffusion transfer recording system, a speeding up of the print speed has been progressing. Consequently, such a problem has arisen that even though thermal energy of a conventional level is applied to both a conventional ink sheet and a conventional image-receiving sheet for printing, it is difficult to obtain a sufficient color density developed. This problem has been improved, for example, by a method of increasing thermal energy at the time of printing, or by a method of making a substrate sheet of the ink sheet into a thin-film to increase efficiency of heat transfer. These actions for increasing thermal energy to be applied to the image-receiving sheet at the time of printing necessitate the image-receiving sheet to have both a responsibility to a low thermal energy at the low density portion and a responsibility to a high thermal energy at the high density portion. Ordinarily, by a method of employing a thermoplastic polymer as a receptor polymer of the image-receiving sheet and also controlling a glass transition temperature (Tg) of the polymer, it has been accomplished to attain both the dye transferring property (generally the lower the Tg is, the higher the transferring property is) and the releasing property (releasability) of the image-receiving sheet from the ink sheet (generally the higher the Tg is, the less the problem of heat seal causes).
Under the above-mentioned measures, it is difficult to attain both the dye transferring property and the releasing property from the ink sheet, in a wide range of temperature. Besides, in a recent dye diffusion transfer recording system, for the purposes of protecting a surface of the image and improving fastness property of the image, a system of laminating the surface of the image with a protective layer has been becoming the mainstream. Consequently, it is also necessary to take care of transferring property of the protective layer to the heat-sensitive transfer image-receiving sheet. The transfer temperature of the protective layer is generally set lower than the dye transfer temperature (at least lower than a temperature necessary to achieve the maximum density), for prevention from thermal diffusion of the image. As a result, there is such a tendency that as the releasing property of the image-receiving sheet from the ink sheet becomes more favorable, transfer of the protective layer to the image-receiving sheet becomes more difficult.
The “‘favorable’ releasing property (of the image-receiving sheet) from the ink sheet” herein used means that any troubles that are called “sticking”, such as generation of peeling noise, failure in traveling (or conveying) of the sheet, and/or occurrence of peeled line in the image, do not occur in the image-receiving sheet. Usually the sticking occurs in the image-receiving sheet, upon when an ink ribbon adheres to the surface of a receptor layer with its binder resin in a printing step, and after the printing, the ink ribbon is stripped off from the image-receptor layer, to cause sticking. If such the sticking occurs, not only a quality of the print image deteriorates, but also especially in the case of a serious sticking, a binder resin of the ink sheet adheres to the above-described surface of a receptor layer. As a result, in the case where an ink ribbon has failed to strip off from both a substrate of the ink ribbon and the surface of the receptor layer, it is in some cases caused such a trouble that an image-receiving sheet is pulled by the resultant ink ribbon, so that the image-receiving sheet is blocked or clogged in a printer without being delivered out from the printer, or a trouble that even though the image-receiving sheet is delivered out from the printer, the ink ribbon is broken.
For resolving these problems, a method of introducing a releasing agent into a surface of the image-receiving sheet has been proposed. Japanese Patents No. 2572769 and No. 2854319 describe releasing agents, such as polyethylene wax, amide wax, and Teflon (registered trade mark) powder, each of which is to be added to a receptor layer of the heat-sensitive transfer image-receiving sheet, and also disclose inventions to enhance releasing property from the ink sheet. However, these patents are silent in disclosure of measures to attain both transferring property of the dye and transferring property of the protective layer.
JP-A-11-321139 (“JP-A” means unexamined published Japanese patent application) describes a method of introducing a carnauba wax into a receptor layer composed of a certain polyester compound. This publication also describes that introduction of the carnauba wax enables to effectively prevent sticking from being occurred and also to improve releasing property from the ink sheet.
JP-A-2005-238748 describes a method of introducing a urethane-modified wax into the image-receiving sheet, thereby to attain both enhancement of transfer density and releasing property from the ink sheet. Still, this publication is silent in a method of attaining both any of the above-described properties and transferring property of the protective layer.
In the image-receiving sheets as described in these patent publications, however, even though it could have been accomplished in some degree to attain both transferring property of the dye and releasing property from the ink sheet, and further transferring property of the protective layer by regulating addition amounts, achievement of compatibility has not yet reached to a revel enough to be satisfied.
In order to dissolve the above-described problems, it is necessary to develop a technique by which a releasing agent effectively works owing to function of a high thermal energy at a high density portion, thereby to attain both the aforementioned releasing property and the transferring property of the protective layer according to a low thermal energy applied at the time of transfer of the protective layer.